Progression through the cell cycle is driven by the ordered accumulation and disappearance of cyclins, a family of proteins which bind and activate serine/threonine kinases collectively known as cyclin-dependent kinases , or CDKs. The B-type cyclins bind to the CDK, Cdc2, and phosphorylation of critical substrates by Cdc2/cyclin B kinase complexes is thought to initiate the dramatic cellular rearrangements characteristic of mitosis. Entry into mitosis involves coordinated changes in distinct subcellular compartments. To achieve this coordination, nuclear and cytoplasmic Cdc2/cyclin B complexes undergo precise activation by upstream regulators, which are themselves restricted to particular subcellular compartments. This proposal is aimed at elucidating the mechanistic basis of regulated Cdc2/cyclin B localization, its relationship to the activity of the Cdc2/cyclin B complexes, and its role in promoting and coordinating entry into mitosis. We have recently demonstrated that Cdc2/cyclin B1 complexes shuttle continually in and out of nuclei throughout interphase, and that this nuclear shuttling is controlled by Cyclin B1 phosphorylation. Accordingly, aim I of this proposal is directed towards determining the role of Cdc2/cyclin B1 nuclear shuttling in coordinating the G2/M transition in both Xenopus oocytes and in vitro cell cycle extracts derived from Xenopus eggs. The experiments in Aim II are designed to identify the kinase(s) regulating cyclin B1 nuclear shuttling. Finally, in Aim III, we will explore the molecular basis for cytoplasmic tethering of the cyclin B1 relative, cyclin B2, throughout the cell cycle. Collectively, these experiments should provide a detailed understanding of how the subcellular localization of the key mitotic regulators Cdc2/cyclin B1 and Cdc2/cyclin B2 is regulated and how this regulation impinges on cell cycle transitions.